Asteroid 253 Mathilde, a Near-Earth Asteroid photographed by NASA's NEAR (Near Earth Asteroid Rendezvous) mission in June 1997. Mathilde is about 60 km in diameter and orbits in the asteroid belt between Mars and Jupiter.

Asteroids are rocky or metallic objects, most of which orbit the Sun in the asteroid belt between Mars and Jupiter. A few asteroids approach the Sun more closely. None of the asteroids have atmospheres.

Asteroids are also known as planetoids or minor planets.

THE ASTEROID BELTThe asteroid belt is a doughnut-shaped concentration of asteroids orbiting the Sun between the orbits of Mars and Jupiter, closer to the orbit of Mars. Most asteroids orbit from between 186 million to 370 million miles (300 million to 600 million km or 2 to 4 AU) from the Sun. The asteroids in the asteroid belt have a slightly elliptical orbit. The time for one revolution around the Sun varies from about three to six Earth years.

The strong gravitational force of the planet Jupiter shepherds the asteroid belt, pulling the asteroids away from the Sun, keeping them from careening into the inner planets.

THE KIRKWOOD GAPS
The asteroid belt is not smooth; there are concentric gaps in it (known as Kirkwood gaps). These gaps are orbital radii where the gravitational forces from Jupiter do not let asteroids orbit (they would be pulled towards Jupiter). For example, an orbit in which an asteroid orbited the Sun exactly three times for each Jovian orbit would experience great gravitational forces each orbit, and would soon be pulled out of that orbit. There is a gap at 3.28 AU (which corresponds to 1/2 of Jupiter's period), another at 2.50 AU (which corresponds to 1/3 of Jupiter's period), etc. The Kirkwood gaps are named for Daniel Kirkwood who discovered them in 1866.

HOW MANY ASTEROIDS ARE THERE?

Gaspra, Asteroid #951.

There are about 40,000 known asteroids that are over 0.5 miles (1 km) in diameter in the asteroid belt. About 3,000 asteroids have been cataloged. There are many more smaller asteroids. The first one discovered (and the biggest) is named Ceres; it was discovered in 1801.

THE SIZES OF ASTEROIDS

Asteroid 4 Vesta, the brightest asteroid and the fourth largest. Vesta is the only asteroid that can be seen without a telescope (it is sixth magnitude).

Asteroids range in size from tiny pebbles to about 578 miles (930 kilometers) in diameter (Ceres). Sixteen of the 3,000 known asteroids are over 150 miles (240 km) in diameter. Some asteroids even have orbiting moons.

CERES: THE LARGEST ASTEROID
Ceres is the largest of the asteroids. It was the first asteroid ever discovered (by the Italian astronomer Giuseppe Piazzi on January 1, 1801). Ceres is the size of the state of Texas! It is so huge in comparison with the other asteroids that its mass is equal to over one-third of the 2.3 x 1021 kg estimated total mass of all the 3,000 cataloged asteroids. Ceres is about 578 miles (930 kilometers) in diameter. Ceres is now considered to be a dwarf planet.

Asteroids can be pulled out of their solar orbit by the gravitational pull of a planet. They would then orbit that planet instead of orbiting the Sun.

Astronomers theorize that the two moons of Mars, Phobos and Deimos, are captured asteroids.

ORIGIN OF THE ASTEROID BELT
The asteroid belt may be material that never coalesced into a planet, perhaps because its mass was too small; the total mass of all the asteroids is only a small fraction of that of our Moon. The total mass of all the asteroids is about 2.3 x 1021 kg ); our moon's mass is 7.35 x 1022 kg; the asteroids' mass combined is about 1/30 of the mass of the Moon. A less satisfactory explanation of the origin of the asteroid belt is that it may have once been a planet that was fragmented by a collision with a huge comet.

TROJAN ASTEROIDS
Trojan asteroids are asteroids that orbit in gravitationally stable Lagrange points in a planet's orbit, either trailing it or preceding it (these places are where the gravitational attraction of the Sun and of the planet balance each other). Jupiter has the most Trojan asteroids; Mars also has some. Achilles was the first Trojan asteroid found. The asteroids preceding Jupiter in its orbit were named for Greek heroes; those following Jupiter in its orbit were named for Trojan heroes.